Structural Dynamics of a Southwestern Pine Forest under Chronic Human Influence

Abstract

Abstract This paper examines the impact of several synchronous disturbances, both natural and anthropogenic, on the development of the Navajo ponderosa pine forest in the Chuska Mountains of the American Southwest during the past several hundred years. The specific objective is to determine the relative importance of historical factors, grazing, and fire regime alteration, as well as climate fluctuations, in changing the forest pattern. Evidence indicates that the open, parklike pine forest of the past underwent major structural change to become significantly denser and younger today. A comparison of reconstructed stand-age structure of fifty years ago with present structure quantifies this change. GIS analysis comparing aerial photomosaics confirms the change at the landscape scale. A history of Navajo pastoralism, beginning by the eighteenth century, suggests that grazing pressure fluctuated but often exceeded carrying capacity. Fire scar analysis reveals that the early regime of light, frequent fire experienced a sudden and persistent drop in frequency around 1830, when grazing animals caused a reduction in fine fuels. Instrumental records for the Colorado Plateau since 1900 indicate, and tree-ring indices for the past several hundred years confirm, a strong warming trend and anomalously wet decades early in this century; these are conditions favorable for ponderosa growth and regeneration. Significant human impacts were felt in the Navajo forest by 1830, while forest structure shifts did not appear until early this century. The timing of the dramatic shift to a more dense, even-aged forest suggests that climate variation was also an important agent of change.